Duplicate lot Number

We are using E1 9.1 & in our environment We have set up , which allow Same lot in 2 different branch plant . Also at the same time we generate the lot number for Work order completion & Purchase receipt based on the ( Lot MM/DD Nmbr) , having this setup we have noticed some of the time system generating the Same lot number for 2 different Work order completion & Same lot number for  Work order completion & Purchase order receiving ...   How come in a day two different transaction some time generating same lot number ????  most